L37
The Canadian M inerala g i st Vol.33,pp. 137-151(1995)
GOMPOSITIONAND OCCURRENCEOF ELECTRUM ATTHE MORNINGSTAR DEPOSIT, SAN BERNARDINOCOUNTY, GALIFORNIA: EVIDENCEFOR REMOBILIZATION OF GOLD AND SILVER
RONALD WYNN SIIEETS*, JAMES R. CRAIG em ROBERT J. BODNAR
Depanmen of Geolngical Sciences, Virginin Polytechnic h stitate and Stale (Jniversity, 4A44 Dening Hall, Blacl Arsrnacr Elecfum, acanthiteand uytenbogaardtite have been examined from six depthswithin the tabular quartzt calcite sockwork and breccia-filled veins in the fault-zone-hostedMorning Star depositof the northeasternMojave Desert, Califomia. Six distinct types of electrum have been identified on the basis of minerat association,grain moryhology and composition. Two types, (1) p1'rite-hostedand (2) quartz-hostedelectrum, occur with acanthite after argentite and base-metalsulfide minerals in unoxidized portions of the orebody; the remaining forr types, (3) goethite-hostedelectrum, (4) electnrm cores, (5) electrumrims and (6) wire electrum,are associatedwith assemblagesof supergeneminerals in its oxidizedportions. Pyrite- hosted quartz-hostedand goethite-hostedelectrum range in compositionfrom 6l ta 75 utt.7oAu and have uniform textures and no zoning. In lower portions ofthe oxidized ore zone, electrum seemsto replacegoethite and occursas small grains on surfacesof the goethite.Textural evidencefavors supergeneremobilization of Au and Ag, which were depositedas electrum on or replacinggoethite. This type of electrumis identical in appearanceand compositionto prinary electrum,In the upper portions of the oxidized zone,secondary electum occursas a gold-rich rim on a core of elechum and as wire-like grains,both with acanthiteand uytenbogaardtite.Such secondaryelectrum contains from 78 to 93 wt./o Au. Textural relations and asso- ciated minerals suggestthat the primary electrum was hydrothermally depositedand partially remobilized by supergene processes. Keywords:electrum, gold (Au), silver (Ag), uytenbogaardtite,supergene, remobilization, Moming Star deposit,Califomia. SovtrvIARE Nous avons examin61'61ectrum, l'acanthite et l'uytenbogaarditeproveuant de six niveaux dans le stockwerk i quaru t calcite et le systdmede brbchesen veines du gisementde Moming Star, mis en place da:rsune zone faill6e dansle secteur nord-est du d6sertde Mojave, en Califomie. Il s'y trouve six types d'6lectrum, identifiables par l'association de min6raux, la morphologiedes grains et leur composition:(l) dlectrumpi6g6 dansla pyrite, (2) 6lectrumpi6g6 dansle quartz, ces deux types associ6si l'acanthiteremplagant l'argentite et aux sulfires desm6taux de basedans les partiesnon oxyd6esdu gisement, (3) 6lectrumassocid d la goethite,(4) noyau des grains,(5) bordure des grains et (6) 61ecfiumen filament, ces quttre demiers associ6saux assemblagesde min6rauxsupergbnes dans les partiesoxyddes du gisement.L'6lectrum dansla pyrite, le quartzou la goethite a une teneur en Au compriseentre 6l *75Vo (Wids) et une texture uniforme, sanszonation chimique. Dans les partiesinf6rieures de la zone oxyd&, 1'6lectum sembleremplacer la goethite,et est dispos6een petits grains sur les surfaces de celle-ci. D'aprbs l'6vidence texturale,il sembley avoir eu remobilisationsupergdne de Au et Ag, et ddpositionsous forme d'6lectrum sur (ou en remplacementde) la goethite. Cette g6n6rationsecondaire d'6lectrum possEdeune apparenceet une compositionidentiques i l'6lectrum primaire. Dans les partiessup6rieures de la zoneoxyd6e, l'dlectrum secondairese pr6sente sous forme d'un liserd de bordure sur un coeur d'6lectrum, ou bien en filaments, les deux en associationavec acanthite et uytenbogaardtite.L'6lectrum secondairea une teneur en Au entre 78 et 93Vo(poids). D'aprbs les relations texturaleset les rnin6raux associ6s,l'6lectrum primaire a 6t6 ddpos6dans un milieu hydrothermal et a 6t6 partiellement remobilis6 pax processussupergbnes. (Traduit par la R6daction) Mots-cl6s:€lectrum, or (Au), argent(Ag), uytenbogaardtite,supergbne, remobilisation, gisement de Moming Star, Califomie. * Presentaddress: Depaxment of Geology, 1-26Earth ScienceBuilding, University of Alberta Edmonton,Alberta T6G 2E3. 138 TTIE CANADIAN MINERALOGIST Iltnorucnon approximately7.3 million metric tonnes of ore aver- agng 2.057 g/tonne(Ausburn 1988).The mine was Chemical models used to describethe distribution operatedintermittently in this century, with the last of gold and silver minerals between oxidized and episodeof mining occurringfrom 1987to mid-1991; unoxidizedportions of gold depositsremain equivocal heap-leachextraction of gold and silver continues for many deposits.Gold and silver remobilization by today. Additional subeconomicprecious-metal miner- low-temperatureoxidizing groundwateris suggested alizationoccurs thro0ghout a seriesof prospectpits in as an important supergeneprocess leading to the KewaneeHills areq south of the mine, and along secondaryenrichment of someprecious-metal deposits the Sunnysidefault, to the west of the mine @g. lB). (Emmons 1,972,Boyle L968, 1978,Larkin et al. The IvanpahMountains are underlainby Jurassicto 1974). Also, paning-stagehydrothermal activity may Cretaceousweakly peraluminous,magnetite-series remobilize gold and silver (Barton et al. 1978). granitic plutons of the Teutoniabatholith, which Increasedfineness of electrumduring mineralizationis constitutespafr of the Mesozoicmagmatic arc that commonlycited as evidencefor remobilizationof gold intruded the southernportion of the Mesozoicforeland andsilver (F.--ons 1912,MacKay 1944,Koshman & fold-and-thrustbelt (Burchfiel & Davis 1972,1975, Yugay 1972), but recent studies (Mann 1984, 1988,Beckerman et al. 1982,Barton et al. 1.988). Stoffregen1986) convincingly arguefor remobi- Subeconomicporphyry copper-molybdenumand lization of gold and silver without an increasedfine- relatedskam mineralization is associatedwith portions nessof the depositedelectrum in certain chemical of the batholith in the adjacentmountains (Ntiamoah- environments.Other investigatorshave suggestedthat Agyakwa 1987), and tin, tungstenand copper skarn- supergeneremobilization of gold and silver may gpe mineralization surroundsthe Ivanpah Mountains actually lead to a decreasein finenessofthe electrum Qlewett 1956, Thompson 1978). Detailed description in specificcircumstances (Webster & Mann 1984). of the regional geological setting and associated Other mineralsof gold and silver may be produced mineralizationis given in Sheets(1995). during late-stagehydrothermal or supergeneoxidation of precious-metaldeposits. For example,uytenbo- Gsolocy oF THEMoRNTNG Sran Dnposn gaardtite(AgrAuS), a rare silver-gold sulfide, has beenreported in a small number of deposits,where it The areaaround the Monring Star depositis under- occurs as a late-stagehydrothermal or supergene lain by coarse-grained,equigranular Ivanpah granite phase@arton et al. L978,Dunn et al. 1985,Castor & of Jurassicage (Fig. 1B). Mineralization occurs in Sjoberg 1993). Uytenbogaardtitehas recently been foliated to mylonitic Ivanpahganite situatedbetween identified in the upper levels of the Morning Star tle Morning Star and Sunnysidethru$t faults, both deposit(Sheets et al. 1988,1989).Stability relation- orientedroughly N20'W/35"SW. Each fault is marked ships for uylenbogaardtite,acanthite and electrum in by a zone of ductile deformation(mylonite and proto- the supergeneenvironment are not currently available, mylonite) overprintedby brittle deformation. but low-temperature(100'C) hydrothermalequilibria Precious-metalmineralization at the Morning Star predict the formation of uytenbogaardtite and deposit is hostedby the Ivanpah granite in the upper acanthiteby sulfidationof electrum(Barton I 980). plate of the Morning Star thrust fault (Figs. 1B, C). This paper describesthe distribution and chemistry The fault crops out at the mine site as a zone of fault of precious-metalmineralization in the vicinity of the gouge consisting of bleachedand rounded granite Morning Star deposit and places constraintson con- fragmentsof variable size surroundedby anastomos- ditions attending gold and silver remobilization. As ing foliated, clay-rich selvages.Very rare galenaand previously stated,tle remobilizationof gold and silver chalcopyritehave been found in fault-gougesamples, may occur either with an increasein finenessof elec- but gold and silver mineralization is truncated at trum (Mann 1984) or with a constantor slightly the gougelayer. The upper edge of the Morning Star decreasingfineness (Webster & Mann 1984, diorite dike. which underliesthe thrust fault. is nun- Stoffregen1986). The Morning Star depositis unusual cated by the Morning Star fault, and fragmentsof in that textural and chemical evidence support both altereddiorite are found in samplesoffault gouge.A trends in the compositionof electrum, at different series of gouge-filled E-W- to ENE-WSW-nending levels within the oxidizedhorizon. vertical to sleeply north-dipping unminslalized sftug- tures cut the orebody,Morning Star fault and Morning LocAnoN ANDllrsrony Star diorite. Displacementon the E-W structures appearsto be left-lateral normal oblique-slip, with The Morning Star deposit is located on the south- maximumthrow of 5 meterson eachE-W structure. easternflank of the Ivanpah Mountains, northeaslern Disseminatedbase- and precious-metalmineraliza- Mojave Desert, California, approximately 100 km tion at the Morning Star deposit is structurally southwestof Las Vegas,Nevada (Fig. lA). Proven controlled within a tabular zone of quartzI calcite reservesat the Morning Star mine, in 1987, were stockworkvein svstemand silica-floodedbreccia units ELECTRIJMAT TIIE MORNING STAR DEPOSIT t39 30m E$EH T*EEgr ls EgE.fT * ' -ral r 3 :-f F"i'B': I ; g -EF:" I sry I , 9rFs t I /t fit-J , c) 3:sE , , EA5H I z I N t , i'=,F o L , r$ t.1 1 .N+ isg $ I (J rl t iiq aoB ,l EI.E 9 | (t-- --! x .ir ., : F? tr F S='E L--r-l' I ug- 5 ;ts9E d oE v -8 a ao! N EtrFB e I eei€ EB ->l EEB; d i - a I €;a; Qe y s gE {;E (dq t ^ rsfEs io ? E€ E EEE€ bae o.b'E A H € ? s,ii.3 (D I pt.sE bo ;o6F: (D - o= Fi !.1 =d o2 c sBEFs'EA;F ilF!eH E* EErr ll.m .Ed€t F nffiEEH/.t < Ho S€ h i;ca$g.E3t€ € ;= b.Ets i3 -e-- tr z ii: -a t E;!#H l.! j. -' x g$1q fi ir. " SFd F€ \)ff,'. l! FI lrX r a t t r E**,EE ttrttii t, 'a | | | , a a itiiitt ! lt, EF*.eE tttaattatraaa tlttta lt ! €a=;E aattaa t t t \r r t a J av r, EF F€ B I ta a t t E4$tB S=E5 E4 E 8"<3€ Hg{€t ,a 1,s d fi.1\/r1o"r fEEgS tttttt 'i'gE€€ N.. tr+r tr E€.F*E HSg-e 8 :; HgE 140 TI{E CANADIAN MINERALOGIST subparallelto the Moming Star thrust fault @ig. lC). intersectionof principal setsof fracfures. The tabularorebody is truncatedto the north and south Although the ore horizon forms a continuousregu- by unminela[izedclay-gouge-filled E-W high-angle larly shapedtabular body (Frg. lC), concenhationsof structures,and to the east by the Moming Star fault. gold within the ore horizon have a complex distri- Horizontal thickness of the ore horizon increases bution owing to the strucfurally conholled nature of below the zone of intense oxidation. which occurs the deposit. Silver concentrations,where data are down to the 1340 and 1370m elevation.and corre- available, generally mirror gold concentrations.The spondsto the present-daywa[er table. Standingwater relationship between gold and silver assayvalues is in abandonedmines and perchedwater tablesare near consistentfor both oxidized portions and unoxidized neutral to slightly acidic (pH rangesfrom 6.5 to 8.0). portions of tlte deposiqbut gold and silver concenfra- The coincidenceof oxidation with the present-day tions are depletedin the zone of oxidation compared groundwatertable has been noted in other gold to the unoxidized zote. Also, a slight difference in deposits of the northeasternMojave Desert (pers. Au/Ag existsbetween the oxidized ore and the unoxi- corlm., J. Cline, 1992). dized ore. Au/Ag values for the oxidized portions of Two st4gesof mineralization have been identified the ore body rangefrom 0.005to 1.150and have an on the basis of petrographicstudies and results of ayerage(+1o) of 0.143+ 0.169(N = 116),whereas electron-microprobeanalyses (Sheets 1995). An early An/Ag valuesfor unoxidized portions of the orebody stage of hypogene q'oartz,calcite, barite, fluorite, rangefrom 0.010to 5.000and average0.340 t 0.540 pyrite, chalcopyrite,galena, sphalerite, tetrahedrite, (N = 164).Rock-chip-based geochemical surveys acanthite after argentite and electrum occurs below around the KewaneeHills historic workings exhibit the zone of oxidation, and a later stageof calcite, similar relationshipsbetween gold and silver as covellite, digenite (t chalcocite), goethite, acanthite, oxidized surface-sampleswithin the mine. uytenbogaardtite,native bismuth, elecfum, Pb, Zn and Concentationsof gold in the Moming Star mine have Cu carbonatesand Cu oxidesoccur within the zone of a strong geochemicalcorrelation with concenffations oxidation. Partia[y oxidized remnantsof assemblages of Ag, As, Cu, Mo, Pb and Bi, and a moderatecorrela- of primary minerals are present in most of the tion with levels of Zn alLdCd (Sheets1995). Similar oxidized ore. Mineralization is confined to quartz t correlationswere seenin rock-chip and soil samples carbonateveins, stockworkzones and brecciatedunits. aroundthe KewaneeHills area(Forgerson 1974, Early-formed quartz and pyrite veins are commonly Byington 1988).With the exceptionof the As and Sb deformed.A secondepisode ofquartz and pyrite was anomalies,all trace-elementcorrelations have been precipitated as open-spacefillings, and base- and accountedfor by minslals associatedwith gold and precious-metalmineralization typically fills small silver (discussedbelow). For a more detaileddescrip- openingsin this secondepisode of quartz.Cavities and tion of the geology, mineralization and geochemisny vugs are typically lined ly euhedralcrystals of quartz of the Morning Star deposit,see Sheets (1995). and filled with coprecipitated pyrite, galena, sphalerite,chalcopyrite, tetrahedrite, acanthite and Occunnm,IcEAND Covposrnou or electrum. Quartz-absentcalcite veins occur later than Pnsqous-ME"rALMNERALS main-stage precious-metal mineralization and only contain galena and chalcopyrite.Carbonate veins are Electrum and silver-gold sulfide minerals have more abundantin upper levels of the deposit, where been examinedpetrographically and analytically at mafic dikes or mafic-mineral-bearingfault gougewere seven locations in the vicinity of the Morning Star encountered.Factors controlling the intensity of deposit (Figs. 18, C). Four sampleswere taken from precious-metalmineralization include fracturedensity, the high-grade ore horizon vdthin the open-pit mine presenceofpyrite with galen4 sphaleriteand chalco- (4700,46f,0,4480and 4450 bench samFles), and two pyrite (or the oxidized equivalen*), and silicification. samplesrepresent high-grade ore from drill core Pervasive propylitic alteration occurs in both the (MSE-23 and MSE-87-04) beyondthe current extent upper and lower plates of the Morning Star fault and of the mine workings. Another sample with micro- envelopsthe area of mineralization. The intensity of scopicgold (K-Hills) was takenfrom an incline in the the propylitic alteration decreasesmarkedly away southernKewanee Hills area. Although this latter from the deposit and is truncatedto the north by an samplehas beenplotted on Figure lC by its elevation E-W high-angle strucfure. Argillic t sericitic alter- and distance from the Morning Star fault, it is a ation occurs only adjacentto mineralizationin the surface sample and has been completely oxidized. upperplate. Alteration of the Morning Star fault gouge gamFledescriptions are given in Sheets(1995). and unmineralizedE-W structuresis also considered Tbe chemical composition of electum, silver sul- to be relatedto the episodeof argillic t sericitic alter- fides and uytenbogaardtitewas determined using ation (Sheets1995). Quartz veins and veinlets are CamecaSX50 and ARL-SEMQ electronmicroprobes. prevaleat throughout the tabular ore horizon and at Details of analytical proceduresand standardsare structurally higher levels of the upper plate at the given in Sheets(1995). Grains smaller than three ELECTRTJMAT TIIE MORNING STAR DEPOSIT r41 TABIA 1. COUPOSIrION OF TYPB OF EITECAEUM micrometersin the long dimensionwere analyzedas a AT TEE UOBNING STAB DEPOSTT grain composite.Larger grains of electrum were ana- beam-sizes. TY?EOF TCPEOF COIJ) CoNTINT (wt.g) lyzed in multiple spots using various ETiECTBUM On'E Every attemptwas madeto analyzelarger grainsin at BANOE ![EANtla least two placesin the central portion of the grain and Pyrlt€-hostsd inddl'€d 83-?5 88.3 3 8.4 qusltz-host€d usddlred €IJ-72 88.7i 9.' at least two spotsnear the marytn. The chemicalcom- goqtbtte-hgt€d odauzsd e2-10 88.7 r 2.8 position of electrum is summarizedin Table 1 and @F odauzed 8!-12 88.1! 2.8 lln oddtzed ?8-93 83.4 i 5.3 shown on Figure 2. Representativecompositions of wtE odaHsoal. 81-90 85.4r 2.? acanthiteand uytenbogaardtiteare summarizedin Table 2. The complete data-setis given in Sheets (1995), and a copy is availablefrom the Depositoryof UnpublishedData" CISTI, National ResearchCouncil of Canada"Ottawa, Ontario KIA 0S2. 10 IA (D ra electrnmcores .e, 8 pyrtte-hosted electrnm N=40 c N=39 36 L 94 tr .=, 7. a gBOJ quar'lz-hosted electrum electrumrlms clt N=fil tl N=26 EH b4 z72 (A o) 9,8 wlredectm N=27 tr6l Cri b4 ! zt2 70 t0 q) 7A E0 q) 100 Wt. 7o Aa Wt. VoAu Flo. 2. Composition of the six types of electrum. Each datum representsthe average compositionfor a given grain, core,rim or wire structures.N is the numberof grains, rims orwires analyzed. r42 THE CANADIAN MINERALOGIST TABIA 2. BEPBESENTATIVE ELECTBON-IIICBOPBOBE DATA FOR ACANTEITE AND T'TTENB@AANDTITE AT TE8 IIOANING STAB DEFOSIT Normallzed rt. t Uols t Au Ag Au AS ACANTBIIE K-sttta-t 0.@ 8:r.3? 3.46 13.19 0.00 0t.41 4.38 83.21 AgrSSCuo.t3S K-lrtxE-2z o.(Xt 85.65 r.09 13.28 o.{Xt 64.89 r.4O 3S.n Agr.gcoOOls 4?CX)-1 6.?5 78.98 t.4 t2.86 2.88 01.62 1.00 33.?0 AUODAg133CuOSS 470o-t2 4.97 78.W 3.88 12.S6 2.08 5S.68 5.0i1 9!1.23 AuofiAglecuo.$S UTTENB@AABDTI:IE 4?00-9 8.28 57.23 1.99 11.52 13.89 4S.8 2.99 i13.58 Aoo.mAerusC"orrg 4?00-11 26.8? 56.5? 4.5? 11.99 t2.t2 41.s 0.50 93.?9 AuOr4Ag2SCuoj9Sz Electrum has been divided into six distinct fypes bandssubparallel to the mylonitic foliation. The main basedon grain morphology, associationwith primary stageof pyrite depositionformed euhedralisolated and secondaryminslals, and composition.These types crystals or interpenetrating aggregatesof cataclasti- of electrumare listed in Table 1. cally deformed subhedral grains of pyrite. It is the main-stagepyrite that is spatially and temporally asso- Electrum in unortdizedore ciated with precipitation of base-metalsulfides. In samplesfrom drill hole MSE-23, main-stagepyrite is Primaq/ hypogeneelectrum occurs with quartz and cut by 0.1- to 1.0-cm-widefine-grained quartz veinlets pynte in unoridized portions ofthe orebodybelow the free of base- and precious-metalminerals. Individual 1340to 1370meter elevation.Rare grains ofelectrum grains of pyrite can be correlatedacross these late were found in pyrite rernnantsin partially oxidized veinletsof quartzwith little or no lateral displacement. upper portions of the deposit.Precious-metal mineral- In other samples,vein edgesconsist of fine-grained ization is restrictedto quartz (t carbonate)veins with quartzthat gradesinward into open vugs encrustedby abundantpyrite and galena, lesser chalcopyrite and later euhedral quarl:z.The vugs commonly contain sphaleriie,and minor acanthiteand tetrahedrite.Early base-metalsulfides with pyrite, electrum and acan- pyrite-bearingquartz veins consist of fine-grained thite. bands of pyrite depositedat the vein-host rock contact.The early-depositedpyrite and quartz exhibit Pyrite- ho sted electntm abundantevidence of deformation.manifest as brittle fracturing of pyrite and undulatory extinction, Electrum in the unoxidized ore commonly occurs subgrainstructures and grain-sizereduction of tle with galenain inclusionsand fracturefillings in main- envelopingquartz. Some early pyrite-bearingveins stagepyrite (Figs. 3A, B). Pyrite-hostedgrains of contain pyrite fragments cataclastically shearedinto electrum range from less than I to approximately Elpynte Elelectrum@l galena@ quartz FIc. 3. Line drawingsfrom photomicrographsillustating the typical occurrenceofpyrite- hostedelectrum with galenain main-stagepynte. A. Electrum and galenainclusions in pyrite. B. Electrumand galenaalong fracturesin cataclasticallydeformed pyrite. FTFCTRUM AT TIIE MORNING STAR DEPOSIT L43 70 pm in the long dimension,with the vast majority of Qunrtz- ho s t ed eIe c t rurn grainsbeing about 10 pm long. Grain shapeis largety controlledby the host pyrite and its fracture geometry. Electrum also occurs in unoxidizedportions of the Pyrite-hostedelectrum in polished sectionshas a uni- orebodyin quartz surroundingpyrite from main-stage form silvery white color acrossindividual grains. pyrite-bearing veins. Quartz-hostedelectrum is Minor acanthite was found with pyrite-hosted elec- slightly later in the paragenesisand occurs inter- trum only in the caseofpyrite surroundedby abundant stitiatly befweeneuhedral crystals of quartz and along base-metalsulfides. No silver was detectedin any healed microfractures in quartz. Quartz-hosted galenaassociated with electrum and acanthite.Silver electrum and electrum at the pyrite-quartz intetface sulfide in the primary minsl6l assemblagewas origi- are generallyfiner grained (less than 5 pm across) nally depositedas argentiteand has converledto the than electrumin pyrite, but exhibits the sameuniform low-temperaturepolymorph, acanthiteoupon cooling, texture and appearanceas observedin pyrite-hosted as inferred on t}te basis of abundanttwinning in the electrum.Also, electrumin quartz and on the surfaces acanthitepseudomorphs. Aithough twinning in acan- of pyrite is not as commonas pyrite-hostedelectrum. tlite has beeninterpreted as being indicative of inver- sion from the high-temperaturepolymorph argentite, Electrurnin oxidizedore Taylor (1969) showedthat acanthitegrown at low temperaturesalso can be twinned; hence,twinning The oxidized portions of the orebody consist alone is not definitive of temperatureof formation. mainly of goethite, digenite (t chalcocite), covellite, Gold-to-silver ratios throughout the unoxidized cerussite,smithsonite, malachite and azurite in quartz portions ofthe tabularorebody suggest the presenceof t carbonateveins. Goethite occurs as a pseudomorph additional silver-bearingminerals, but no other after pyrite and with digenite sunounding chalco- argentiferous minerals were found. The only other pyrite. Typically, oxidation is not complete, so that silver-bearingmineral found was a single grain of Ag- remnantsof pyrite and chalcopyrite persist within bearingtetrahedrite. these pseudomorphs.Heavy-metal carbonateseither Ftc. 4. Photomicrographsof goethite-hostedand wire electrum. A. Goethite-hostedelectrum interstitial to goethite after pyrite pseudomorphs.In A, the elec- trum is bright white, goethite is light to dark gray, and the host quartz is black. B. Goethit€-hostedelec- trum replacing goethite that partially replacespyrite. Iu B, the electrum is bright white, goethite is light gray with low relief, pyrite is light gray with high relief, and the host quartz is darker gray. C. Wire electrum with associatedcovellite, acanthiteand uytanbogaardtite.In C, tle electrum is bright white, covellite is dark gray and bladed, acanthite and uytenbogaardtiteare mottled shadesoflight gray, and the host quartz is black. The grain to the right of the wire electrumconsists of a central zone of mixed acanthiteand uytenbogaardtitewith an outer zone of acanthiteand bladedcovellite. Scalebars are 150 Pm long for both A and B, and 50 pm long for C. 144 TIIE CANADIAN MINERALOGIST form pseudomorphsafter their perspectivesulfide 4700),but havenot beenfound in surfaceexposures in minerals or occur as fracture coatingsand fillings. the KewaneeHills. One grain of electrum with a rim Goethiteand malachiteare the most commoncoatings less than 2 pm thick also was found in drillhole along open fractures and joints. Covellite also forms MSE-87-{4 in the relatively unoxidized portion of surfacecoatings along open fracturesin quafi. These the orebody. Electrum cores, which range from 6 to fractures typically cross-cutthe tabular ore horizon. 140 pn long, are composedof homogeneoussilvery All covellite is blaubleibender(blue-remaining) covel- white silver-rich electrum, similar to pyrite-hosted, lite, on the basisof optical identification. quaxtz-hostedand goethite-hostedelectrum. Grain-size distribution of electrum cores also is skewedtoward Goethite-ho s ted electrum coarser sizes, with an averagegrain-size larger than either pyrite- or goethite-hostedelectrum. Electrum Electrum occurs in the oxidized zone as fracture rims, which range in averagethickness from <1 to fillings and inclusions in goethitepseudomorphs after 41 pm" have a brilliant yellow-gold color in polished pyrite, interstitially between goethite pseudomorphs section comparedto other types of electrum. Rim (Fig. 4A') and embaying goethite or finely disse- thicknesshas a bimodal distribution between thin minaled on goethite surfaces(Frg. aB). Electrum that rims, B 100 .Fr rtq) E60 €) o) Be 40 Ag *i F 20 t0 15 20 2s 30 35 40 45 50 55 60 65 70 75 EO E5 Distance in micrometers Frc. 5. Typical exampleof the occurrenceand compositionof a thick rim of electrumon a core grain. A. Photomicrographof the tliit rim of elecfum and the core zone.The electrumrim has an irregulm bright white appearanceand containsinclu- sionsof covellite, dark gay and bladed.The core zonehas a uniform bright white appearance.The entire grain is encapsu- in latedin quartz Olack). B. Electron-microprobetraverse for Au, Ag and Cu acrossthe electrumcore and thick rim shown A. Indivibual analyseswere performed with a l-pm beamsize, and the traversewas conductedwith a 0.4-pm.step size. 146 TIIE CANADIAN MINERALOGIST toward gold-rich compositions(Fig. 2). No difference similar to that of electrumrims, in the rangebetween in chemical compositionwas detectedfor electrum 8l and 90 wt.VoAu, with one wire giuio U"ing that occurs on the surface of pyrite or which formed 94 wt.VoAu. The composition of wire*electrum is inclusions and fracture fillings in pyrite. Also, pyrite- skewed toward Au-depieted values below the mean hosted electrum shows a uniform appearanceand composition. The mean value of wire electrum is chemical composition acrossindividual grains. essentiallyidentical to the meanvalue for rim elec- Quartz-hostedand goethite-hostedelectrum have the trum. Electrum composition is generally uniform same range and mean co_mpositionas prite-hosted tbroughoutindividual wire structures,compared to the electrum(Table l, Fig.2). No chemicaldistinction inegulardistributionobservedinthickrimj. exists between electrum found as inclusions or frac- ture fillings in goethite,interstitial to goethitepseudo- silver and silver-gold sulfides morphsoor embaying goethite. Goethite-hostedelec- trum also has a uniform chemical compositionfor Coexisting acanthiteand uytenbogaardtitehave individual grains.In core zones,the rangeand median beenfound only within thick rims of elJctrum orursso- compositionof electrumare similar to thosefound in ciated with wire electrum.Acanthite was found in pyrite-hosied,quartz-hosted and goethite-hostedelec- sample K-Hills, which contains goethite-hostedelec- trum, but core zonesare skewedtoward slightly lower trum and no uytenbogaarOtite.a typical occurrence Au content.Electrum rims aremore Au-rich than elec- for silver + goid sulfide minerals includes anhedral trum cores, with rim composition generally gteater massesof intergrown acanthiteand uytenbogaardtite, than 78 wLVoAu. Electrumrims are skewedtoward which commonly contain laths and roJettes of Ae-enri9led compositions(Frg. 2). Two rims have a covellite. These ciusters of silver + gold sutfide and composition less than 74 wt.VoAu (Fig. 2). Both of individual grains of acanthiteor uytenbogaardtiteare theserims are lessthan 2 pm thick and formed around surroundedby wire electrum(fig. 4C) or 6ick rims of large core-typegrains. Thin rims generallyhave gold electrum. Seiondary acanthiteis untwinned, and so contentsat or below the median composition of rim probablyformed at a low temperature. e-l9ctp, which may reflect difficulty in analyzingthe The compositionof the electrumis slightly depleted thin rims without obtaining X-ray scatleringfrom the in gold for wire and rims intergrown wiin Uotn acan- electrum core. Alternatively, the difference may be thite and uytenbogaardtite,as olposed to wire or rims real and reflect diffusion of Ag between cores and without visible silver t gold sdddes. The composition rilos or precipitation of subsequentlymore gold-rich of electrum rangesfrom 80.0 to 85.3 wt.ZoAu in tle electrum as the rims precipitated. These latter presenceofsilvertgotdsulfidesandrangesfrom8l.0 hlpothesesge not likely correc! becauseno correla- io 91.8 wt.VoAu where oo silver-goldsulfides are tion exists between electrum composition in core present.Acanthite compositionranles from 78.5 to grainsand correspondingrims. Also, elementalX-ray 19.5 wt.Eo Ag in the presence o1 electrum and mapsshow a6 2qningofgold andsilverbetween elec- uytenbogaardtite(Table 2). For acanthitecoexisting trum coresand rims acrossthe sharpcore-rim bound- with electrum but not uytenbogaardtite,the composi- aries (Sheets1995). Thus, the slight enrichmentin Ag tion of acanthiterangei ftom 82.5 to 86.5 wt.% Ag of thin rims relative to thick rims is likely a result oi (Table 2). Goethite-dostedelectrum coexisting wid analyticaldifficulties. An exampleof the analytical acanthiteis slightly Au-rich comparedto the mean difficulties can be seen in the thick rim traverse of value for goetlite-hosted electrum.The silver t gold Figlre 5B, where analyses of electrum rims and sulfidessJem to be sulfur-rich,but all s1ye1+ lold enclosedcovellite laths reveal intermediatecomposi- sulfides and wire or thick-rim electrum are found in tions of electrum. The one grain of electrum in the open vugs and fracturescontaining covellite. Because unoxidized ore-zonethat developeda rim (sample Cu appears-due in all analyses,ttre sunu enrichmentis MSE-87*04) has the samedistribution of Ag-rich likely to intergrowthsof covellite.The composi- core (?1 wt.Vo Au) and Au-rich rim (S4 wt.Vo Au) tion of uytenbogaardtite is consistent with those describedabove for oxidizedsamples from the higher reportedby Barton et al. (1978), who noted sulfur levels within the Morning Star deposit.Electrum cores enrichmenrin supergeneor Cu-rich uytenbogaardtite. for any given core-rim-pair are compositionally Acanthiteafter argentitethat occursin qiartz wgs homogeneols(e.g.,Fig.5B). Thin rims alsoappear to with electrumand base-metalsulfides is essentiafy be compositionallyhomogeneous, but fewer analyses pure (containing 86 to 87 wt.Vo Ag), with less than of each thin rim were performed.Thick rims, on the b.OSm.qo combinedAu and Cu. No iulfur enrichment 9t!9r._nan{,are compositionally heterogeneous,with was observedin acanthiteafter argentite. idivi_{ual seementsvarying by up to 12 wt.VoAu Thus, the chemical compos-itionsof goethite- (Ftg. 5B).However, no consistentchange in composi- hosted, quartz-hostedand pyrite-hostedele,itrum and tion was noticedalong traversesaway from the core electrum cores are esseniially identical. Also, no zone' chemical distinction exists for Ag-rich electrum Wire electrum has a distribution of compositions (pyrite-hosted,quartz-hosted, goethid-hosted and core F'J.ITCTRT]MAT TIIE MORNING STAR DEPOSIT r47 electrum types) with depth along the tabular ore dissolution of the enclosedelectrum. Additional horizon. With the exceptionof electrum rims (dis- goethite-hostedelectrum occurs as large grains cussedabove), there is no relationshipbetween grain embaying goethite or as fine grains depositedon the size of electrum and chemical composition.Electrum surfaceof goethite. Theselatler textures suggestthat rims and wire electrum morphologieshave similar remobilization of Au and Ag and precipitation of chemical compositions,and are considerablymore elecrum during late-stagehydrothermal or supergene Au-rich comparedto cores, pyrite-hosted, qnartz- oridation occurredwith little or no changein the com- hostedand goethite-hostedelectrum. position of elecrum. Thus in the highest levels of the deposit,remobilization of Au and Ag causedincreased DlscussIoN finenessof electrum (rim and wire morphologies)and is associatedwith silver t gold sulfide minerals,but in Two distinct stagesof electrum mineralization lower levels of the deposit,the remobilization of Au occur in the Morning Star deposit. The principal and Ag was essentiallyisochemical. stagesof electrum deposition are primary hypogene Uytenbogaardtitecan form in equilibrium with and secondary,either supergeneor late-stage,low- acanthiteup to 113oC(Graf 1968,Barton 1980). Thus, temperaturehydrothermal. Within thesetwo stagesof the assemblageAu-rich electrum + acanthite + deposition,multiple episodesof electrumprecipitation uytenbogaardtitemust have formed during the waning probably occurred.Temperature estimates for primary stageof hydrothermalprocesses or by supergene elecfrummineralization range from 280 to 315oCfrom processes.A supergeneorigin is favoredbecause of fluid inclusions and electrum - argentite - pyrite - the textural overlapwith mineralizationformed during sphaleritethermometry (Sheets 1995). Primary hypo- oxidation of the Morning Star deposit. For example, geneelectum was depositedin rwo stagesfrom fluids wire and rim electrum occur with intergrown moving up along the Morning Star fault (Sheets1995), acanthite,uytenbogaardtite and bladedblaubleibend.er both stagesspatially and temporally related to pynte covellite. This assemblage,and especiallythe presence and galenaprecipitation. The earliest electrum and of blaubleibenl.ercovellite, is consistentwith a super- argentite(now acanthite)were depositedcontempora- geneorigin. Furthermoreothe zone of oxidation corre- neously with main-stagepyrite and base-metal spondsto the present-daywater table at the deposit, sulfides. Silver-bearingtetrahedrite may also be part which arguesfor fairly recent supergeneoxidation. In of this assemblage,as one grain of tetraledrite was addition, only one Au-rich rim and no uytenbo- found. Trace-elementgeochemistry also suggestsa gaardtitewere found below the zone of oxidation, strong correlation ,urong As, Sb, Ag and Au in the suggestingthat remobilizationof gold and silver unoxidized ore (Sheets1995). No other As and Sb occurredowing to descendingfluids from the surface mineralshave beenidenffied in the primary ore. The insteadof fluids rising from below. Late-stage secondepisode of primary electrum occurs with hydrothermalfluids would rise along the Morning galena in cataclastically deformed pyrite and quartz Star fault and should have remobilized or oxidized vugs. The two episodesof primary elecftum are textu- significant quantitiesof the deep ore. No evidenceof rally distinct but are chemically and mineralogically this deep oxidation was found. Although mineral- similar, and may form one continuous episode of ogical and textural evidence supports the supergene electrumdeposition. formation of silver t gold sulfide minerals and the Textural evidencefrom the oxidized portions of the remobilization of Au and Ag, a late-stagehydro- orebodysupport the late-stageremobilization of at thermal origin cannotbe completelyruled out at this least someof the gold and silver. In the uppermost time. portions of the orebody, electrum is present as a Present-daywaters at the Morning Star mine, higher-finenessrim arounda lower-finenesscore. The Sunnysidedecline and rain water at the minesitehave core zoneis chemicallysimilar to all stagesof primary a pH between6.5 and 8.0. The mineral assemblage hypogeneelectrum. Lower in the oxidized orebody, malachite+ cerussite+ smithsonite,common in exten- the electrum also is chemically similar to primary sively altered samples,is consistentwith thesepH electrum, but occurs in various textural relationships values (Mann & Deutscher1984). Thus if recent with goethite.Much of the goethite-hostedelectrum is supergenesolutions are responsiblefor oxidation and probably primary electrum that was not remobilized. remobilization of preciousmetals at the Morning Star Evidencefor oxidation without remobilization deposit,these solutions should have been near-neutral includeselectrum with cerussiteas inclusionsin to alkaline. Locally, acidic conditions may have goethite after pyrite pseudomorphsand electrum prevailed owing to the breakdownof pyrite and other in pyrite remnantswithin goethite.These grains of sulfide minerals" but the abundanceof carbonate goethite-hostedelecfrum are texturally and chemically minerals, especiallyin the upper portions of the identical to electrumwith galenathat occursas inclu- deposil probably reactedto neutralizethe acidic solu- sions in pyrite (pyrite-hostedelectrum). Replacement tions. At depth within the tabular ore horizon, where of tlese sulfide minerals mav have occuned without the abundanceof sulfide mineralsincreases and that of 148 THE CANADIAN MINERALOGIST ttre carbonateminerals decreases"acidic conditions bilization of Au and Ag. Bisulfide speciescould may prevail locally. only be responsiblefor transportingAu and Ag in Gold and silver are soluble and transportedin oxy- lower portions of the depositowhere reducing condi- genatedaqueous solutions by organic complexes tions prevail and total dissolved sulfur in solution is (Boyle er al. 1975,Baker 1979,Vlassopoulos el a/. higher (Webster1986, Gammons & Barnes1989, 1990),halogen complexes (Krauskopf 1951, Cloke & Vl4ssopoulos& Wood 1990,Benedetti & Bouldgue Kelly 1964, Seward1976, Mann L984,Zotov et al. 1,99r). 1986), hydroxide complexes(Zotov et al. 1982, Websler (1986) experimentally showedthat the Vlassopoulos& Wood 1990) and S-donorligand solubility of electrumis a function of composition. complexessuch as HSO, and S2Or2-(Goleva et aI. Electrum containing50 at.VoA9(f'4.6 wt.7oAu) has a 1970,Plyusnin et al. L981,Webster 1986, Gammons lower solubility as thiosulfate complexesthan gold- & Barnes1989, Renden & Seward1989). Ifprecious- rich elecffum or pure gold. Webster(1986) interpreted metal mobilization took place at the Morning Star theseresults in terms of the formation of a mixed- mine under conditionssimilar to present-daycondi- metal complex [(Au,Ag)(S2O)f]. Whether a mixed- tions, organic complexesare probably of no conse- metal complex actually forms is beyond the scopeof quence,because vegetation is sparseand only occurs this paper. However, when Au and Ag are dissolved in alluvial valleys.Chloride complexesare only signif- from electrumwith a compositioncontaining between icant at pH values less than 5 (Cloke & KelTy 1964, 60 and 70 tnt.VoAtt, the resulting electrum should be Seward 1976), and hydroxide complexesare not depositedwith little or no chemical separation significant if other dissolved species,particularly betweenAu and Ag. Thus, precipitationof secondary sulfur-bearingspecies, arc present(Zotov et at. 1982, electrumshould occur with a composifioasimilm to Vlassopoulos& Wood 1990).For thesereasons, that of the initial primary electrum.Primary electrum neither halogen or hydroxide complexes,re con- (pyrite-hostedand quartz-hostedelectrum and elec- sideredto be imFortantmechanisms of tansport at the trum cores)at the Morning Star deposithas a median Morning Star deposit. composition of 68 wt.VoAu, which is identical to Thus, for the proposedsupergene conditions at the goetlite-hostedelectrum. Goethite-hosted electrum Morning Star deposit, thiosulfate would form occurs in various textural relationshipsthat resemble metastablesulfir speciesthat could remain in solution both primary electrum and electrum depositedfrom in high concentrations(Goldhaber 1983, Williamson the remobilization of Au and Ag. Solubility and & Rimstidt 1992) and could account for the remo- transport of Au and Ag by thiosulfate complexesare acanthite tr uytenbogaardtite o gold-rich electrum r primary electrum cores 20Vo'q=r---'-r*-=-=S3,=a=.==l20%o Ag Au Ftc. 6. Chemistryof coexistingelectrum (diamonds),acanthite (circles) and uytenbogaardtite(squares) plotted on the 100"C isothermalsection showing Au-Ag-s in weight vo, after Barion (1990), whoseplot is given in moiar units, which have beenconverted to unis of weight in this paper.Dashed line representsthe equilibrium mineral assemblagesat 100.C. The bll9k rectanglerepresents the compositionof the primary electrum,and the arrows indicate composition:alchaages due to sulfidation of electrum-The black circle (a) and square(u) show the ideal compositionsof acanthiteanrl uytenb6gaardtite, respectively. ELECTRI,TVIAT THE MORNING STAR DEPOSIT 149 consistentwith both texfures.Precipitation of sec- blagesin the oxidized portions of the orebody'Pyrite- ondary goethite-hostedelectrum may have occurred hosted and quartz-hostedelectrum represetrtprimary by reductionof thiosulfatecomplexes, local decreases electrumdeposited by hydrothermalprocesses. in pH or sorption onto goethite and electrum surfaces Elecuum cores and much of the goethite-hostedelec- (Webster1986, Schoonen et al. 1992). trum have chemicalcompositions simil2l to that of In the highest levels of the Moming Star deposit, primary elecfum, and are consideredto representpn- electrum occurs as gold-rich rims on core zones and mary electrum that remainedunaltered by oxidation. wire structureswith silver t gold sulfide minerals.In Goethite-hostedelecfium that embaysgoethite or was light of the phaserelationships determined by Barton depositedon goethite surfaceshas texturesindicative (1980), the higher finenessof the electrum can be of supergeneremobilization of Au and Ag, but accountedfor by sulfidation of original compositions exhibits chemical compositions similar to that of of electrum (Fig. 6). Reactionof primary electrum primary electrum. Remobilization of Au and Ag by with sulfur produces the assemblageacanthite + thiosulfatecomplexes can produce secondary goethite- electrum, and further sulfidation would eventually hosted electrum with no chemical separationof Au produce the assemblageacanthite + uytenbogaardtite and Ag, and should form abundantmetastable com- + electrum (Fig. 6). Each addition of sulfur and plexesfor the inferred supergenefluids at thelr4oming changein mineral assemblageform electrum with Star deposit. Electrum cores with rims of higher- high concentrationsof Au, driving the compositionof fineness electrum and wire electrum occur in the the electum to 84.0 vtt.VoAu in equilibrium with ideal uppermostlevels of the Morning Star depositwith compositions(Barton aianthite and uytenbogaardtite.Rims of gold-rich acanthiteand uytenbogaardtite 'ng 1980).Goethite-hosted electrum coexis with acan- electrum and wire structures are consistent with thite is richer in Au than the meanfor goethite-hosted sulfidation of primary electrum to forrn silver t gold electrum. which is consistentwith the sulfidation sulfide minerals.Thus, within the Morning Star theory and the phaserelationship determined by deposil remobilizationof Au and Ag occursisochemi- Barton (1980). Equilibrium is commonly not main- cally and with gold enrichment,each occurring at tained during supergeneplocesses, and no atiempt is different levels within this one deposit. made to imFly that equilibrium conditions prevailed during supergeneoxidation of the Morning Star ACKOWI.EDCETVDNTS deposit, but compositionsfor rim and wire electrum have a mean value of 84.4 wt.VoAu, which is con- The authors thank Jack Jordon, Jr. and the sistent with experimentalphase-equilibria. Castor & Vanderbilt Gold Corporationfor accessto the Sjoberg(1993) also found compositionsof electrumin Morning Star mine, financial support and permission the assemblageacantfiite + uytenbogaardtite+ elec- to publish theseresults. Todd Solbergprovided assis- trum, in tle Bullfrog depositof Nevada,that agree tanie with the electron-microprobeanalyses. The with the phaseequilibria of Barton (1980). Transport manuscript benefitted from critical reviews by S. by thiosulfate complexesmay also produce gold-rich Castor and an anonymousreferee, and editorial electrum in rim and wire morphologies (Webster & handling of R.F. Martin; although thesereaders Mann 1984,Webster 1986).As silver reactedto form improved this paper,all errors and misundentandings acanthite,and silver and gold reactedto form uytenbo- arethe responsibilityof the authors. gaardtite,the thiosulfate complexeswould be destabi- lized and precipitate gold-rich electrum owing to the RtrRm{cEs preferential loss of silver. Further data regarding /z the stability of acanthiteand uytenbogaardtiteduring AuSBURN,K.E. (1988): Geology of theMoming Star mine. GeologicalJourneys in theSouthern supergeneprocesses, especially plocesses related to ThisExtended Land, Basinand Range (D.L. Weideand M.L. Faber,eds.). thiosulfate solutions, are required to ver$ this final Geol. Soc.Am., CordilleranSection, Field Trip hlpothesis. GuilebookT6. CoNcLUsIoNs BA@, W.E.(1979): The role of humicacid in thetransport of gold.Geochint Cosmochim. Acta 42, fu5-649, Within the structurally controlled high-grade ore BARroN,M.D. (1930):The Ag-Au-S system.Econ. GeoI. horizon at the Morning Star deposit, electrum grains 75,303-3L6. have been divided into six types basedon associated G.E., CAPo,R.c.' minerals and grain morphology. Theseinclude (1) BArrLBs.D.A., Dssour, Devts, S.R., HeNsou' R.B.' pyrite-hosted (2) quartz-hostedelecffum occurring CHRrsrBNsEN,J.N., and & TRM, H.E. (1988): Mesozoiccontact minerals por- Mlcrm,soN. C.J. with base-metalsulfide in unoxidized metamorphism in the western United States. ft tions ofthe orebody,and (3) goethite-hostedelectrum, Metamorphism and Crustal Evolution of the Western (4) electrumcores, (5) electrumrims, and (6) wire United States(Rubey Vol. VII) (W.G. 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